We are interested in the characterization of two proteins of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF). These proteins with apparent molecular weights of 120 and 155 kilodaltons (Kd) have been shown to be important immunologically and have been implicated as virulence determinants. A monoclonal antibody (Mab) reactive with the 120-Kd antigen was reactive by ELISA with a virulent strain isolated from Montana (Sheila Smith strain), but not with a strain of lesser virulence isolated from North Carolina (Morgan strain). This Mab was capable of passively protecting mice from a potentially lethal challenge of the Sheila Smith strain, but not from challenge with the Morgan strain; a Mab reactive to the 120- Kd protein of both strains did passively protect against both. SDS-PAGE analysis revealed that the 120-Kd protein of the Morgan strain migrated faster than did that of the Sheila Smith strain, while the relative migration of the 155-Kd protein was slower. An N-octyl-beta-D-glucopyranoside (OGP) extract of rickettsiae, enriched for the 120- and 155-Kd proteins, was used to immunize guinea pigs. After challenge, guinea pigs which had been vaccinated experienced decreased height and longevity of fever and less severe scrotal reactions as compared to guinea pigs inoculated with buffer alone. A Mab reactive to the 155-Kd protein was administered along with oxytetracycline to guinea pigs infected with R. rickettsii. These guinea pigs had fewer symptoms of RMSF as compared to groups of guinea pigs treated with either water, oxytetracycline, or Mab alone. We recently reported the cloning in Escherichia coli of the gene encoding the 155-Kd protein. A lysate of this recombinant (JM107(pGAM22)) was used to vaccinate guinea pigs; another group of guinea pigs was vaccinated with a lysate of E. coli harboring the vector alone (JM107(pUC8)). The guinea pigs were challenged with viable R. rickettsii. As compared to the guinea pigs which received the JM107(pUC8) lysate, guinea pigs vaccinated with the recombinant lysate (JM107(pGAM22)) had fewer symptoms of RMSF.